The advantage of using electronic rather than mechanical fuel management systems has been recognized by the automotive industry. With such electronic fuel management, it is required to generate and provide mass airflow data to the control system to regulate the appropriate fuel/air combustion ratio. In order to accomplish precise fuel control in an automotive internal combustion engine, mass air flow data is determined through utility of a mass air flow sensing device positioned upstream of the intake manifold of the engine. In an engine with pulsing or reversing flow, the fuel management is improved if the sensing device is a bi-directional mass air flow (BAM) sensor to be able to measure both in-flow and out-flow of air, so that errors due to monitoring flow reversals in the manifold can be avoided.
A typical bi-directional mass air flow sensor generally consists of a thin film heater element and four thin-film sensor resistors on a thin membrane of glass on top of a substrate. The heater element and sensor resistors typically are formed of platinum (Pt), and the substrate typically is a micromachined silicon wafer. The sensor resistors are connected in a Wheatstone bridge circuit configuration to convert the sensed temperature difference into a corresponding voltage. The substrate material is removed from a small area beneath the heater element and the sensor resistors, and left under the area of the sensor containing the interconnects and mechanical support. The glass over the area where the substrate has been removed commonly is referred to as the xe2x80x9cwindowxe2x80x9d. The heater element is energized to produce a temperature at the center of the window that is considerably higher than ambient temperature; this arrangement results in a temperature gradient from the center of the window to the edges of the window. The high thermal coefficient of resistance (TCR) of the thin-film resistor material causes the resistance of the sensor resistors to change in proportion to the temperature change in the area of the window containing the sensor resistors. With proper calibration of the sensor, a gas caused to flow across the sensor, perpendicular to the length of the sensor resistors, will cause the temperature gradient on each side of the heater element to change in a manner that allows the direction and mass flow rate of the gas flow to be determined.
However, a bi-directional mass air flow sensor must be thermally balanced in the center of the window, or the sensor output will drift with time at low flow rates. By the term xe2x80x9cthermal balancexe2x80x9d it is meant that the temperature gradient must not change with time for a consistent air flow, or, that the change in gradient on one side of the sensor is cancelled by a compensating change on the other side of the sensor. Previous methods attempting to achieve this required thermal balance have included the addition of a metal framing around and extending over the edge of the window, as well as techniques to place an etch stop into the silicon substrate to define the edge of the window.
Now, according to the present invention, a novel circuit arrangement is provided for thermally balancing a bi-directional mass airflow sensing device. The invention provides for a bi-directional mass air flow sensing device for measuring air flow, comprising a bridge circuit coupled across a voltage potential, wherein the bridge circuit comprises: a first side including first and second temperature dependent sensor resistors connected in series and disposed on a thermally insulative substrate window in line with an air flow and arranged such that relative to a first direction of air flow, the first sensor resistor is upstream of the second sensor resistor; a second side in parallel with the first side and including third and fourth temperature dependent sensor resistors connected in series and disposed on the thermally insulative substrate in line with the air flow such that relative to the first direction of air flow, the third sensor resistor is upstream of the fourth sensor resistor; and, a temperature dependent balance resistor connected between the first and second temperature dependent sensor resistors on the first side of the bridge circuit.
The balance resistor physically is positioned between the heater and the contiguous sensor resistors on the sensor window, and, electrically is connected between upstream and downstream sensor resistors on one side of the Wheatstone bridge circuit. A shunt resistor may be placed in parallel with the balance resistor. In this arrangement, some of the current that normally would flow through the balance resistor now flows through the shunt resistor. If the voltage across the bridge circuit remains the same, then changing the amount of current flow through the bridge changes the total resistance as measured externally. If the shunt resistor is divided into two separate resistors, then the change in TCR can be adjusted differently for the upstream and downstream sensor resistors on one side of the bridge circuit.